The present application claims priority under 35 U.S.C. xc2xa7119 of European Application No. EP 00118972.9, filed on Sep. 1, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety. The present application also incorporates by reference herein in its entirety the disclosure of German Patent Application No. 199 43 075.6, filed on Sep. 3, 1999.
1. Field of the Invention
The invention relates to technical ceramics and to porous aluminum oxide structures and processes for their production. These production processes also relate to those processes used for the production of mesoporous filtration membranes, more coarsely structured intermediate layers or filter substrates, as well as catalytic converter substrates.
2. Discussion of Background Information
Filter modules made of Al2O3 have been available for some time and may typically comprise a construction of several layers having graduated pore sizes. Although coarsely porous ceramic oxide filter substrates usually comprise corundum (xcex1-Al2O3), and optionally glass as binding agents, predominantly solutions of transitional aluminas (xcex3-, xcex4-, or xcexa-Al2O3) have been used for the separating layers and the transitional aluminas are usually deposited via a sol/gel process in the mesoporous range of 20-60 nm, which is advantageous for application techniques. The transitional aluminas used in the sol/gel process comprises precursors of Al2O3. However, a construction of pure corundum is desired due to the more disadvantageous chemical and thermal stability of transitional alumina as compared to xcex1-Al2O3.
It would be technologically advantageous to produce the more coarsely structured intermediate layers using the sol/gel process, that is, layers intermediate between a separating layer comprising 80 nm particles and the substrate having pore sizes of 1-2 xcexcm; however, no previously used sol/gel process has been able to produce Al2O3. structures having a sufficient pore size of more than about 50-100 nm and a sufficiently high porosity of greater than about 30% by vol., and preferably greater than about 40% by vol. Typically, the known sol/gel processes produce only more finely grained, mesoporous structures, which comprise the above-mentioned transitional phases of Al2O3. When these structures are ignited at high temperatures in order to enlarge the pores, a considerable increase in pore size does not occur until the transition to the thermodynamically stable corundum phase and, therefore, it is connected to a sudden collapse of the porosity to low levels reducing its usability.
Further, powder technologies using Al2O3 powder, instead of precursors of the Al2O3 powder, cannot be used for producing porous layers of the aforementioned type, since layers of powder, which are formed by dip coating in powder slips, have a very high compacting density due to the grain sizes of 0.1-1 xcexcm in the unsintered state of the powder and do not allow a connection of pores in a desired size range of 100-500 nm with porosities greater than about 40% by vol.
Heretofore, it has not been known how to produce mesoporous Al2O3 structures of high porosity comprising corundum and having an average pore size of 20-60 nm.
Nor has it been known how to use in sol/gel-processes other solutions known for producing sintered, highly porous Al2O3 structures having desired larger pore sizes from about 50 to about 1000 nm.
To make the sol/gel process usable for producing intermediate layers of commercial precursors (such as DISPERAL(copyright), a boehmite made by Condea Chemie, Hamburg, Germany), a considerable material transport must be allowed at high porosity and must remain at a high porosity level during ignition. Since the boehmite has primary particle sizes of 2-7 nm and agglomerate sizes of 30-60 nm, usual annealing conditions will lead to smaller pores as desired here. On the other hand, sol/gel-processes originating from such boehmites are known for small particle sizes and a strong surface curvature of the particles leading to a high sintering activity and enhancing the dense sintering. Within the known theories for solid phase sintering, it cannot be expected, therefore, that it is possible to overcome the above-mentioned problems in the production of mesoporous corundum structures and of structures having pore sizes of 50-1000 nm, while maintaining an evenly high porosity. Thus, according to valid theories, considerable pore growth is related to grain growth which is always parallel to a considerable reduction of residual porosity. Thus, according to Coble, J. Appl. Physics, vol. 32(5), pp.787-792 (1961), which is incorporated by reference herein in its entirety, the sintering process comprises three stages:
An initial stage is characterized by the increase of the sintering necks from zero to an area equivalent to xc2xd of the cross section area of the particles. The process is accompanied by a small percentage of shrinkage which already represents a reduction of porosity from the originally typical 40-50% (at a relative initial density of 50-60%) to 30-40%, without a considerable growth of the particles even being possible. During annealing, the porosity already reduces in the initial stage of the sintering process, and without any grain or pore increase, to values that mark the limit of usability for many highly porous products.
An intermediate stage begins when the first moderate, grain growth and a change in the shape of the pores start the transformation into a structure having pores and larger amounts of grain limits. The overwhelming amount of porosity is open; during the sintering process, porosity reduction correlates with shrinking cylindrical pore channels with an overall low grain growth. See Johnson, J. A. Ceram. Soc. 53(10), pp. 574-577 (1970), which is incorporated by reference herein in its entirety, who assumes a constant particle size for his model of an intermediate stage. See also Greskovich u.a., J. Am. Ceram. Soc. 55 pp. 142-146 (1972), which is incorporated by reference herein in its entirety, whose measurements show in MgO-doped Al2O3 a grain growth from 300 to 660-850 nm, while simultaneously cutting porosity in half.
A final stage starts with the transformation to closed porosity and corresponds with an increased grain growth and an enlargement of the average pore size, with a considerable reduction of porosity.
The present invention provides porous aluminum oxide structures comprising xcex1-Al2O3 that, at a high porosity, have mesoporous pore structures having average pore sizes in the range from about 20 to about 60 nm. A mesoporous structure as referred to herein is a structure having a pore size between about 2 to about 60 nm. The present invention also provides porous aluminum oxide structures comprising xcex1-Al2O3 that, at a high porosity, have pore structures in larger average pore sizes up to about 1000 nm. Here, pore sizes are defined as average xe2x80x9ceffectivexe2x80x9d pore diameters that result from conventional methods of mercury-porosimetrical measurements. Since real open pore structures cannot have ideal spheric or cylindrical forms, no real xe2x80x9cdiameterxe2x80x9d is present in the pores. Thus, the pore diameter results as the effective value based on known geometrical models.
The porous aluminum oxide structures are producible by powder techniques as well as by sol-gel processes. Additionally, porous aluminum oxide structures are producible over the entire range of pore sizes of xcex1-Al2O3.
The present invention relates to a porous aluminum oxide structure comprising Al2O3 and Zr, the structure having an open porosity greater than about 30% and an average pore size from about 20 to about 1000 nm, wherein the Zr has a concentration which, expressed as ZrO2 based on Al2O3, constitutes less than about 5 weight % of the weight of the Al2O3.
The porous aluminum oxide structure preferably has an open porosity greater than about 40% and the Al2O3 comprises xcex1-Al2O3.
The porous aluminum oxide structure preferably can have an average pore size from about 20 to about 60 nm or can have an average pore size from about 50 to about 1000 nm.
In another aspect of the present invention, the porous aluminum oxide structure comprises xcex1-Al2O3 and has an open porosity greater than about 30% and an average pore size of from about 20 to about 60 nm, and preferably an open porosity greater than 40%.
In yet another aspect of the present invention, the porous aluminum oxide structure can have an open porosity greater than about 30% and an average pore size of from about 50 to about 1000 nm, and preferably an open porosity greater than about 40%.
In a further aspect of the present invention, the porous aluminum oxide structure comprises xcex1-Al2O3 and the Zr has a concentration which, expressed as ZrO2 and based on Al2O3, constitutes less than about 5 weight % of the weight of the Al2O3. The porous aluminum oxide structure also can have an open porosity greater than about 30% and an average pore size from about 20 to about 1000 nm, and preferably an open porosity greater than 40%.
In a preferred aspect of the present invention, the porous aluminum oxide structure can have an average pore size from about 50 nm to about 1000 nm, or can have an average pore size from about 20 nm to 60 nm.
The present invention also relates to a porous aluminum oxide structure comprising xcex1-Al2O3, and Zr, wherein the Zr has a concentration which, expressed as ZrO2 and based on Al2O3, from about 0.03 to about 1.5 weight % of the weight of the Al2O3. The porous aluminum oxide structure can also have an open porosity greater than about 40% and an average pore size from about 50 to about 1000 nm.
The present invention also relates to a process for producing a porous aluminum comprising Al2O3 and Zr, the structure having an open porosity greater than about 30% and an average pore size from about 20 to about 1000 nm, through doping alumina or precursors thereof with a doping effective amount of Zr, wherein the Zr, expressed as ZrO2, has a concentration less than about 5 weight % of the weight of the Al2O3, wherein the process comprises: introducing the Zr into the alumina or the precursors thereof, the Zr being selected from at least one ZrO2 powder, a solution of a Zr precursor, or mixtures thereof, by admixing the Zr with the alumina or the precursors thereof, forming a green body, drying the green body, and sintering the dried body to produce the porous aluminum oxide structure.
In the process, the Zr can be introduced into the alumina or the precursors thereof by admixing the Zr in the alumina or the precursors thereof.
In another aspect of the present invention, the Zr can be introduced into the alumina or the precursors thereof by introducing the at least one ZrO2 powder into the alumina or the precursors thereof through milling a suspension of the alumina or the precursors thereof with ZrO2 balls.
In the process, the green body can be formed by casting the admixture into a mold. In yet another aspect of the present invention, the green body can be formed by dip coating a porous substrate to form a membrane layer.
The sintering can take place at a temperature from about 700xc2x0 C. to about 1600xc2x0 C., preferably from about 850xc2x0 C. to about 1400xc2x0 C., and more preferably from about 900xc2x0 C. to about 1370xc2x0 C.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure.
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.
The porous aluminum oxide structures according to the present invention are preferably provided with a Zr dopant.
The process for producing porous aluminum oxide structures according to the present invention comprises adding a Zr dopant during production in such a concentration that it is less than about 5 weight % expressed as ZrO2 based on the weight of Al2O3.
The production process, according to the present invention, provides porous aluminum oxide structures having an open porosity greater than about 30%, preferably greater than about 35%, and more preferably greater than about 40%. Open porosity is defined as percentage of pores that are not completely closed, i.e., the pores have passageways connected to each other.
The production process, according to the present invention, provides porous aluminum oxide structures having an average pore size from about 20 nm to about 1000 nm, and more preferably from about 20 nm to about 500 nm.
The production process, according to the present invention, provides porous aluminum oxide structures having a Zr content, which, expressed as ZrO2 and based on the weight of Al2O3, is less than about 10 weight % ZrO2, preferably less than about 5 weight %, and more preferably less than about 1.5 weight % ZrO2.
The Zr can be incorporated into the structures in the known manner by using known ZrO2 materials or precursors. ZrO2 precursors are those compounds which are hydrolyzable in water and ultimately form ZrO2 or optionally hydrated ZrO2. These ZrO2 precursors include, but are not limited to ZrOCl2, or Zr(OR)4 where R is alkyl or lower alkyl. Preferably, the lower alkyl can be from 1 to 7 carbon atoms, either straight chain or branched alkyl, including isopropyl and isobutyl. More preferably, R can be CH3, C2H5, C3H7 or C4H9. A preferred Zr precursor is aqueous zirconium oxychloride solution. To incorporate the Zr, the sol/gel processes as well as the powder technological processes can be used.
A particularly advantageous high porosity can be maintained up to high annealing temperatures by using from about 0.03 weight % to about 1.5 weight % ZrO2. Smaller additions reduce the desired effect of stabilization of high porosity in the product, preferably exceeding 40 vol. %; higher concentrations can cause undesired side effects, such as the modification of the chemical behavior of the created products.
The effect of low Zr dopant (ZrO2 after conventional air sintering) noted here cannot be explained and is not equivalent to any other known effects of this addition during the sintering of Al2O3. JP 62-21750, which is incorporated by reference herein in its entirety, has reported that higher amounts of ZrO2 enhance the sintering of corundum. However, K. Bhatia u.a., xe2x80x9cPressure Assisted Sintering of Mixtures of Alumina/Zirconia Powdersxe2x80x9d, 23rd Annual Cocoa Beach Conference, The Am. Ceram. Soc., Jan. 24-29, 1999, Lecture S1-061-99, which is incorporated by reference herein in its entirety, has reported that low additions less than about 1% ZrO2 impede the sintering of corundum. Both results only point, in one case, to a general diffusion accelerating effect, and, in the other case, to a diffusion slowing effect of different ZrO2-concentrations. GB 2,071,073 A and B2, which is incorporated by reference herein in its entirety, is analogous to one of the countless examples that describe the impeding effect of low ZrO2 concentrations to the grain growth of sintering corundum.
In contrast thereto, the present invention provides the conditions for a surprisingly different effect that enables the ignition with advanced pore growth at an almost constant porosity (i.e., reduced densification).
It is believed that Zr or ZrO2 probably contaminates the surface of the still highly porous Al2O3 in such a way that diffusion processes driving the densification are largely decelerated without stopping the leading diffusion processes necessary for the pore growth. The present invention uses Zr or ZrO2 for producing highly-porous Al2O3 sintering products of a defined pore structure in such a way that it is equally usable for powder technical processes using Al2O3 powder as well as for sol/gel processes using precursors of Al2O3.
It should be noted that the influence of the Zr dopant and/or the ZrO2 dopant for the phase transformations to be performed can obviously be different depending on the existing conditions of the sol/gel processes. Vereshtshagin et al., Zh. Prikladnoy Khimii vol. 55(9), pp. 1946-51 (1982), which is incorporated by reference herein in its entirety, reports that Zr4+ has no effect on the corundum yield, i.e., with Zr4+ as well as without it, 88-90% corundum results at 1250xc2x0 C. However, Xue, J. Mater. Sci. Lett. vol. 11(8), pp. 443-445 (1992), which is incorporated by reference herein in its entirety, finds, among other things, an aggravated corundum formation with an increased temperature from 1216xc2x0 C. to 1291xc2x0 C. under the influence of ZrO2 added as oxychloride.
On the other hand, the sol/gel process of the present invention comprising dispersing the above-mentioned boehmite with Zr and/or ZrO2 dopant provides a very high yield of corundum at 1150xc2x0 C. and, starting at approximately 1200xc2x0 C., only corundum is produced which, in contrast to previously known results, indicates a transformation accelerating effect of this type of dopant.
A particular advantage of the present invention is the fact that mesoporous aluminum oxide structures comprising corundum and having intermediate pore sizes from about 20 to about 60 mn can be produced with high porosity greater than about 30%, and preferably greater than about 40%.
Another particular advantage of the present invention is that the structures having the listed characteristics can be produced equally well by a process using powders (the Zr compound is in powder form) as well as by sol/gel processes which use a hydrolyzable Zr precursor.
In accordance with the present invention, the sol/gel processes or the powder processes can produce Al2O3. structures with coarser pore structures, i.e., more coarsely structured intermediate layers or substrates, which are characterized by an open porosity greater than about 30%, and preferably greater than about 40%, by average pore sizes from about 50 to about 1000 nm, and preferably from about 100 to about 500 nm, and by a Zr content of less than 5 weight %, expressed as ZrO2 and based on the weight of Al2O3.
The production of mesoporous Al2O3 structures, particularly of chemically and thermodynamically highly stable corundum (xcex1-Al2O3), presents a particular difficulty in relation to prior art. In the sol/gel-processes, the formation of corundum requires the maintenance of certain minimum annealing temperatures, while alternatively, a quick sintering must be expected when highly sinteractive nanocorundum powder is being used. This quick sintering is due to the high sintering activity, which results from the small curvature radius of the particle surfaces. In both cases, a quick reduction of the open porosity during ignition (calcination, sintering) occurs. The dopant according to the present invention counteracts the above and allows the production of mesoporous structures comprising corundum and having high porosity. Both sol/gel technologies and powder technologies can be used in the present invention.
However, the advantage of a phase structure comprising corundum need not be specifically stressed for more coarsely structured intermediate layers or substrates with pore sizes preferably from about 100 to about 500 nm. Although corundum is also particularly advantageous here, it is readily employable within the range of the present invention by using commercial corundum powder and/or with by igniting sol/gel derived products having dopant at temperatures above the temperatures of corundum formation.
In this regard, in accordance with the present invention, sol/gel processes are particularly advantageous since such structures comprising corundum could not be produced using sol/gel previously.
However, the advantage for many applications, particularly of corundum products, does not limit the utilization of the dopant according to the present invention for other purposes where, e.g., Al2O3 phases other than corundum might be more suitable for catalytic converter substrates. It is within the scope of the present invention to produce Al2O3 products having phase compositions other than corundum, which are stable and have high porosity and optimal pore sizes which can be controlled.
In the present invention, the raw materials can include corundum itself, or corundum doped with Zr and/or ZrO2 or any corundum precursor. A corundum precursor is any aluminum compound which will form corundum. The specification also states that numerous corundum precursors may be utilized in the present invention. The precursors include, but are not limited to, aluminum salts (such as aluminum nitrate, aluminum sulfate, alum, aluminum chloride and the like), aluminum alcoholate (such as Al(OR)3 where R is alkyl from 1 to 9 and preferably R can be CH3, C2H5, C3H7 or C4H9), aluminum hydroxides, (such as Al(OH)3 or aluminum polyhydroxides), boehmite, diaspore, transitional alumina (such as xcex3, xcex4, xcex8, or xcexa-Al2O3 as well as corundum itself.
In accordance with the present invention, the dopant leads to the maintenance of a high porosity during the annealing up to the high temperatures necessary for production of porous products. This results in an additional advantage in that the porous products can be used in high temperature ranges.
The present invention is further described in detail for sol/gel processes and powder technical processes using exemplary embodiments. Here, the examples of powder technical processes are described with a broad range of different corundum material having intermediate particle sizes from about 50 nm to about 1.5 xcexcm.